1. Field of the Invention
The invention relates to a lithographic apparatus comprising a position measurement system and a method to determine a position of a first part of a lithographic apparatus with respect to a position of a second part of the lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) of a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
An ongoing development in lithography is that dimensions of patterns to be applied onto the substrate are reduced year by year to be able to provide, e.g., integrated circuits having a higher degree of integration (i.e., more memory cells or processing capacity per square mm of chip area) and/or a higher bandwidth, i.e., a higher processing speed or a lower access time. Also, in the case of a substrate comprising, e.g., a liquid crystal display panel, smaller pattern sizes will support a higher resolution of the display panel and other design requirements. One of the factors to be able to achieve such high accuracies, and to be able to apply multiple layers of patterns onto the substrate which match with each other, is that alignments in the lithographic apparatus have to fulfill more and more stringent accuracy requirements. Such alignments may include, e.g., alignments of various parts of the lithographic apparatus with respect to each other, e.g., an alignment of a substrate table holding a substrate with respect to the projection system, an alignment of a mask table holding a mask with respect to the substrate table holding the substrate, an alignment of the mask table holding the mask with respect to the projection system, as well as many other examples. A common principle in such alignments is that relative positions are determined, i.e., a position of a first part is determined with respect to a position of a second part. The smaller a size of a pattern to be applied onto the substrate, the higher the requirements on the accuracy of the alignment. In current and future generations of lithographic apparatuses, alignment accuracies of better than 1 nanometer may be aimed at. Attempting to achieve such accuracies, sensitivities for a variety of environmental conditions become extremely high: temperature fluctuations (e.g., due to electric dissipation by electronics, motors, lasers, etc.) may result in, e.g., expansion of constructions in the lithographic apparatus and/or expansion of gasses in the lithographic apparatus or other effects on the measurement system. Furthermore, pressure fluctuations (e.g., due to shocks, movements, etc.) may result in small displacements of portions of the lithographic apparatus or have other effects on the measurement system, etc. A further aspect which plays a role here is that frequently use is made of optical detectors such as interferometers and encoders to measure in a contactless way a distance between various elements or parts of the lithographic apparatus. As an example, when aligning a first part of the lithographic apparatus to a second part of the lithographic apparatus, optical detectors may be applied to measure a position of the first part with respect to a reference, while further optical detectors are applied to measure a position of the second part with respect to the reference. From these positions as measured, a relative position of the first part with respect to the second part may be derived, which information may be used to perform the alignment. A complicating factor here is that in particular interferometers perform a measurement with an outcome that is in some way dependent on a wavelength of an optical beam applied by the interferometer (such a laser source thereof). A wavelength of the beam is however, due to physical laws, dependent on various environmental conditions, such as a temperature and a pressure of the medium (such as a gas) through which the beam travels, but also carbon dioxide concentration and relative humidity. Therefore, also pressure of the medium through which the beam travels will affect a wavelength of the beam. Therefore, temperature, pressure fluctuations, etc., not only influence structures of the lithographic apparatus (such as elements of the first and second parts), but may also affect outcomes of measurements by sensors, such as optical measurement devices. Pressure fluctuations may also result in, e.g., refractive index variations which may lead to position measurement errors. In addition to the above, not only temperature and pressure changes have an effect on an accuracy of measurements in the lithographic apparatus, also changes in humidity, in composition of gas mixtures (e.g., CO2 concentration) and many other examples will have an effect thereupon.
As an example, assume that a part of the lithographic apparatus, e.g., a mounting of a lens comprises a radius of 250 mm. Assume that this part comprises a steel having a thermal expansion coefficient of 12 ppm/K. Then, an expansion of 3 μm per degree centigrade results. Thus, when a desired accuracy is in an order of magnitude of 1 nanometer or better, a temperature fluctuation of 0.33 mK may be allowed at maximum. In practical implementations however, local or global temperature fluctuations or changes which may be orders of magnitudes larger, have been observed. Achieving a temperature stability up to a degree of 0.33 mK is therefore considered problematic, in fact virtually impossible in a current lithographic apparatus. Furthermore in an alignment various parts of the lithographic apparatus may play a role, and therefore the allowable budget in tolerance of 1 nanometer may have to be divided over a plurality of parts, thus tightening the above temperature stability requirements even further.